Re: [PATCH 1/2] arm64: mem-model: add flatmem model for arm64

[Catalin Marinas]

On Mon, Apr 11, 2016 at 12:31:53PM +0200, Ard Biesheuvel wrote:
> On 11 April 2016 at 11:59, Chen Feng <puck.chen@xxxxxxxxxxxxx> wrote:
> > On 2016/4/11 16:00, Ard Biesheuvel wrote:
> >> On 11 April 2016 at 09:55, Chen Feng <puck.chen@xxxxxxxxxxxxx> wrote:
> >>> On 2016/4/11 15:35, Ard Biesheuvel wrote:
> >>>> On 11 April 2016 at 04:49, Chen Feng <puck.chen@xxxxxxxxxxxxx> wrote:
> >>>>>  0             1.5G    2G             3.5G            4G
> >>>>>  |              |      |               |              |
> >>>>>  +--------------+------+---------------+--------------+
> >>>>>  |    MEM       | hole |     MEM       |   IO (regs)  |
> >>>>>  +--------------+------+---------------+--------------+
> >>> The hole in 1.5G ~ 2G is also allocated mem-map array. And also with the 3.5G ~ 4G.
> >>>
> >>
> >> No, it is not. It may be covered by a section, but that does not mean
> >> sparsemem vmemmap will actually allocate backing for it. The
> >> granularity used by sparsemem vmemmap on a 4k pages kernel is 128 MB,
> >> due to the fact that the backing is performed at PMD granularity.
> >>
> >> Please, could you share the contents of the vmemmap section in
> >> /sys/kernel/debug/kernel_page_tables of your system running with
> >> sparsemem vmemmap enabled? You will need to set CONFIG_ARM64_PTDUMP=y
> >
> > Please see the pg-tables below.
> >
> > With sparse and vmemmap enable.
> >
> > ---[ vmemmap start ]---
> > 0xffffffbdc0200000-0xffffffbdc4800000          70M     RW NX SHD AF    UXN MEM/NORMAL
> > ---[ vmemmap end ]---
[...]
> > The board is 4GB, and the memap is 70MB
> > 1G memory --- 14MB mem_map array.
> 
> No, this is incorrect. 1 GB corresponds with 16 MB worth of struct
> pages assuming sizeof(struct page) == 64
> 
> So you are losing 6 MB to rounding here, which I agree is significant.
> I wonder if it makes sense to use a lower value for SECTION_SIZE_BITS
> on 4k pages kernels, but perhaps we're better off asking the opinion
> of the other cc'ees.

IIRC, SECTION_SIZE_BITS was chosen to be the maximum sane value we were
thinking of at the time, assuming that 1GB RAM alignment to be fairly
normal. For the !SPARSEMEM_VMEMMAP case, we should probably be fine with
29 but, as Will said, we need to be careful with the page flags. At a
quick look, we have 25 page flags, 2 bits per zone, NUMA nodes and (48 -
section_size_bits) for the section width. We also need to take into
account 4 more bits for 52-bit PA support (ARMv8.2). So, without NUMA
nodes, we are currently at 49 bits used in page->flags.

For the SPARSEMEM_VMEMMAP case, we can decrease the SECTION_SIZE_BITS in
the MAX_ORDER limit.

An alternative would be to free the vmemmap holes later (but still keep
the vmemmap mapping alias). Yet another option would be to change the
sparse_mem_map_populate() logic get the actual section end rather than
always assuming PAGES_PER_SECTION. But I don't think any of these are
worth if we can safely reduce SECTION_SIZE_BITS.

-- 
Catalin